Upgopher v1.21.0 — encrypted file transfers through tunnels

Upgopher v1.21.0 introduces optional application-layer encryption for browser
file transfers. Enable -encrypt-transfers to encrypt content before it crosses
ngrok, Cloudflare Tunnel, trycloudflare, or another HTTP/HTTPS intermediary.

What is new

• Browser-side encryption before uploads and browser-side decryption after downloads.
• Encrypted transport for files, ZIP archives, text previews, file-search excerpts,
  screenshots, and custom shared download links.
• AES-256-GCM authenticated blocks and per-transfer keys protected with
  RSA-OAEP/SHA-256, implemented with Web Crypto and the Go standard library.
• Upload progress and cancellation, automatic temporary-transfer cleanup, and
  direct-to-disk downloads where supported by the browser.
• A prominent website announcement and updated README, configuration, security,
  and protocol documentation.

Enable it

upgopher -dir ./shared -encrypt-transfers -user admin -pass 'choose-a-strong-password'

Point your existing tunnel at http://localhost:9090 and open its HTTPS URL.
Your browser handles encryption and decryption automatically. The server saves
uploaded files in their original format. Existing access controls still apply.
No additional client installation, database, third-party Go module, or runtime
asset dependency is required. The feature is opt-in; ordinary mode is unchanged.

See docs/security.html#transfers for the security model and operational details,
and docs/transfer-protocol.md for the protocol specification.